I may be missing something. It seems that a player cannot always control which stack a unit joins when moving from one province to another. Units moved from one province to another join the top stack in the new province. My problem is that the player does not control which stack is on top in the target province and thus does not control which stack these units join. It is so annoying when units join the wrong stack at the end of their movement. If the last stack selected in a province would remain at the top of the group of stacks when the player selects a stack in another province it would allow the player to determine which stack a unit joins when moving between provinces.